Hail David Roberts. May he rise on a plinth and rest among the immortals. On Tuesday this Home Office official replied to an MP's bovine demand for a statistic with: "I haven't the faintest idea." Give him the Keats memorial prize for truth as beauty.

Roberts said he did not know the number of foreigners illegally in Britain because, by definition, they were illegal and unlikely to be registered in any meaningful way. Policy was, rather, directed at migratory crooks, gangsters and suspected people traffickers. Yet his admission caused a political explosion. It was a Bateman cartoon: "The civil servant who said he had no idea!" A Commons committee swooned in apoplexy. Its chairman, David Winnick, had to be helped into a BBC studio, where interviewers soothed him and sympathised. The Daily Mail, Express and Telegraph went into paroxysms and devoted entire front pages to the outrage.

Roberts's experience was British government in a nutshell. Immigration policy is based on a wing and a prayer, and is well known to be chaotic down at the frontline. What MPs therefore want is a pretence of omniscience, a statistical comfort blanket on which to suck all day. Statistics imply control. The Home Office is like staff headquarters in the Great War. It does not mind if the battle is won or lost so long as the number of dead is counted. It was for want of a statistic that Charles Clarke was doomed.

After lifting the veil on this pretence, Roberts was roasted. Had he merely recited his home telephone number all would have been well. The following day a panic-stricken minister, Tony McNulty, did just that. He told the Commons that his guesstimate of an answer to the Roberts question was "roughly between 310,000 and 570,000". The figure was clearly meaningless, but his audience sighed with relief. Here was clearly a man in command.

While poor Roberts was acting the foolish servant, elsewhere in Whitehall a wise one was on parade. Elinor Goodman of the Affordable Rural Housing Commission was doing what Roberts was not. She was brandishing a figure, in her case of how many "affordable rural homes" were needed by "poor people" to "sustain communities" and thus forestall "a whole generation being lost to the countryside". These qualifiers prompted every question in the book. Who defines affordable, needed, poor, sustainable, rural, communities, lost or countryside? Goodman was undaunted. The answer, she said, was 11,000 a year.

The figure is the fantasy of some Leninist demographer. It is the social geography of "from each according to his means, to each according to his needs". It confuses a supposed "right" of residents to have their offspring subsidised by the state to live near them - surely the most bizarre of human rights - with a supposed "need" of every community for some timeless socioeconomic mix. Urban neighbourhoods recognise no such feudal socialism, at least not since the advent of right-to-buy.

While it is - in a theme-park sense - upsetting that many expensive country villages are half dead during the week, so are many expensive parts of towns at weekends. This is a feature of an ever more mobile community. Even the Archers are nowadays popping in and out of cars. We do not subsidise cheap housing to protect long-standing residents of Belgravia, where property costs little more per square foot than in parts of the Cotswolds or the south coast.

And what is a "need" for a cheap house? Everyone thinks themselves in need of something better. Country cottages are now Britain's most "wanted" asset. Yesterday's social housing (that is, tied cottages and council houses) are tomorrow's lucrative assets. As for giving certain occupations such as rural doctors and teachers a golden handshake and security of tenure for life, this must be the most extravagant and transient form of social engineering.

The other proposal, to create a ghetto market in homes restricted to "the next generation of local people", denies existing homeowners the value of their property and thus mobility for themselves and their children. It is a crazy wealth tax on the rural poor. It also sends the price of unrestricted properties even faster through the roof.

I see nothing wrong in communities deciding to tax themselves or otherwise control their evolution as they choose. But it cuts both ways. As the new housing minister, Ruth Kelly, apparently believes, nimbyism is a democratic right. People are entitled to vote for whatever social or planning protection they prefer, which is not what Goodman advocates. To imply that those bringing new money and, in many cases, new economic activity to rural Britain are a social evil is leftwing archaism.

The rural England of Goodman's working-class nostalgia was once poor and is becoming rich, as parts of London and other cities were once rich and have become relatively poor. Country houses are desirable, especially for those who want to spend their weekends and their retirement in the country. They are bound to be ever more expensive. I cannot see how a lucky few poor people have a right to state-protected tenure in Alresford, Broadway or Hutton-le-Hole any more than in Notting Hill, Solihull or Hallam.

None of this really matters, because all Goodman was expected to find was a vaguely plausible statistic. She duly hit on the magic 11,000. It is said to equal six new "affordables" per village per year. This is roughly what the government already wants, the annual tithe of a meadow from every English village, to be handed over to builders for six houses plus enough "executive homes" to pay for them.

Eleven thousand happens to be Whitehall's perfect number. It is not too small and not too big, not too round (like 10,000) and not improbably specific. It fobs off every fatuous inquiry. Eleven thousand is thus given as the number of gun crimes per year in Britain, the number of GP practices, the miles of railway track, the last reduction in Scottish unemployment, the number of children excluded from school, the number of jobs "created" by the Olympics and the number of terrorist attacks worldwide.

None of these figures is of any use. They are a factoid code. Eleven thousand of anything is a conceptual sum that can be broken down into vaguely manageable units. Had Roberts given 11,000 as the number of illegal immigrants awaiting deportation - which, for all I know, it is - he would have been cheered to the skies. He could have pointed out that this would mean 256 illegals per police authority, or 17 per London electoral ward. That would have given Downing Street a "deportation taskforce initiative target" and Roberts an OBE.

In truth nobody can have the faintest idea how many poor people would like a country home - probably all of them. Nobody can know how many poor people it takes to "sustain" a village (except in the Windrush valley, where the answer is none). Nor can anyone know how many foreigners are roaming the streets of Britain untagged, unprocessed, unregistered, unwatched and unarrested. It really makes no difference.

But power declares that this will not do. Power wants a fig leaf to suggest it has control and means business. That fig leaf is 11,000.